linux-kernel - [PATCH 7/7] x86: Tell about irq stack coverage

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Date:	Wed, 25 Sep 2013 18:18:06 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	LKML <linux-kernel@...r.kernel.org>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@....ibm.com>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	James Hogan <james.hogan@...tec.com>,
	"James E.J. Bottomley" <jejb@...isc-linux.org>,
	Helge Deller <deller@....de>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	"David S. Miller" <davem@...emloft.net>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: [PATCH 7/7] x86: Tell about irq stack coverage

x86-64 runs irq_exit() under the irq stack. So it can afford
to run softirqs in hardirq exit without the need to switch
the stacks. The hardirq stack is good enough for that.

Now x86-64 runs softirqs in the hardirq stack anyway, so what we
mostly skip is some needless per cpu refcounting updates there.

x86-32 is not concerned because it only runs the irq handler on
the irq stack.

Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
Cc: Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc: Paul Mackerras <paulus@....ibm.com>
Cc: Ingo Molnar <mingo@...nel.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: H. Peter Anvin <hpa@...or.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Paul Mackerras <paulus@....ibm.com>
Cc: James Hogan <james.hogan@...tec.com>
Cc: James E.J. Bottomley <jejb@...isc-linux.org>
Cc: Helge Deller <deller@....de>
Cc: Martin Schwidefsky <schwidefsky@...ibm.com>
Cc: Heiko Carstens <heiko.carstens@...ibm.com>
Cc: David S. Miller <davem@...emloft.net>
Cc: Andrew Morton <akpm@...ux-foundation.org>
---
 arch/x86/include/asm/irq.h | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/x86/include/asm/irq.h b/arch/x86/include/asm/irq.h
index 0ea10f27..879bece 100644
--- a/arch/x86/include/asm/irq.h
+++ b/arch/x86/include/asm/irq.h
@@ -23,6 +23,10 @@ extern void irq_ctx_init(int cpu);
 
 #define __ARCH_HAS_DO_SOFTIRQ
 
+#ifdef CONFIG_X86_64
+# define __ARCH_IRQ_EXIT_ON_IRQ_STACK
+#endif
+
 #ifdef CONFIG_HOTPLUG_CPU
 #include <linux/cpumask.h>
 extern void fixup_irqs(void);
-- 
1.8.3.1

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